EVALUATION OF ALLUVIAL AQUIFERS FOR SMALL-SCALE IRRIGATION IN PART OFTHE SOUTHERN SAHEL, WEST-AFRICA

Aquifers in the sand alluvium of the broad riverine hats (fadama) of t he southern Sahel and northern Sudan climatic zones of West Africa are a major source of water for irrigation essential to crops in bad year s. Annual recharge is from rivers flowing seasonally into the Sahel fr om the wetter south. Landforms which evolved in the mid Tertiary contr ol the modern drainage. From the Pleistocene onwards rivers vigorously eroded the crystalline highlands of the Jos and Air massifs and carri ed the coarse alluvium far downstream. The resulting aquifers are ofte n several kilometres wide and mostly over ten metres thick, sometimes much thicker, and are capped by late Pleistocene loessic soils. Aquife r properties are typical of the mostly coarse unconsolidated sands. Th e alluvial groundwater is reached and abstracted by methods appropriat e in scale to family size farms. The system of farming and irrigating is socially beneficial and economically viable. The paper concludes wi th case studies of three rivers of Bauchi State in Nigeria. The alluvi al aquifers are evaluated and related to irrigation potential, express ed as the percentage of land which may be irrigated using only the gro undwater beneath it.